prefix an _ to the name of an unused variable

Author: catamorphism

src/test/run-pass/deep-vector.rs

@@ -1,5 +1,5 @@
 fn main() {
-    let x = [
+    let _x = [
         0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
         0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
         0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,